Methanol is synthesised in large volumes annually by conversion of a carbonaceous feedstock, usually a hydrocarbonaceous feedstock such as natural gas, into a mixture of carbon oxides and hydrogen. Such a mixture of gases is often referred to as synthesis gas.
The conversion of a hydrocarbon-containing feedstock, such as natural gas, into synthesis gas can be effected by steam reforming.
In steam reforming a mixture of desulphurised hydrocarbon feedstock, such as natural gas, and steam is passed at high temperature, typically at a temperature of from about 600.degree. C. to about 1000.degree. C., and elevated pressure, typically from about 10 bar up to about 50 bar, over a suitable reforming catalyst, such as a supported nickel catalyst. One commercially recommended catalyst which can be used for this purpose uses a mixture of calcium and aluminium oxides as support for the nickel. When natural gas is the feedstock, the principal reaction is: EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2
The reaction products themselves are further subject to the reversible "water gas shift" reaction in which carbon dioxide and hydrogen are produced from carbon monoxide and steam: EQU CO+H.sub.2 O.revreaction.CO.sub.2 H.sub.2
Conversion of the carbon oxides and hydrogen to methanol occurs according to the following reactions: EQU CO+2H.sub.2.revreaction.CH.sub.3 OH EQU CO.sub.2 +3H.sub.2.revreaction.CH.sub.3 OH+H.sub.2 O
These reactions are conventionally carried out by contacting the synthesis gas with a suitable methanol synthesis catalyst under an elevated synthesis gas pressure, typically in the range of from about 50 bar up to about 100 bar, usually about 80 bar, and at an elevated methanol synthesis temperature, typically from about 210.degree. C. to about 270.degree. C. or higher, e.g. up to about 300.degree. C.
A conventional methanol synthesis plant can be considered to comprise four distinct parts, namely:
1. a reforming plant, which produces a mixture of carbon oxides and hydrogen from a hydrocarbon feedstock; PA1 2. a compression stage lifting the carbon oxides and hydrogen mixture to a higher pressure suitable for downstream methanol synthesis; PA1 3. a methanol synthesis section, in which crude methanol is produced from the carbon oxides and hydrogen; and PA1 4. a distillation section, in which the final refined methanol product is produced from the crude methanol. PA1 a) a steam reforming zone, adapted to be maintained under steam reforming conditions and charged with a catalyst effective for catalysis of at least one steam reforming reaction, for steam reforming of a vaporous mixture of the hydrocarbon feedstock and steam to form a synthesis gas mixture comprising carbon oxides, hydrogen and methane; PA1 b) a methanol synthesis zone, adapted to be maintained under methanol synthesis conditions and charged with a methanol synthesis catalyst, for conversion of material of the synthesis gas mixture to a product gas mixture comprising product methanol and unreacted material of the synthesis gas mixture; PA1 c) a methanol recovery zone, adapted to be maintained under methanol recovery conditions, for recovery of a crude methanol product stream from the product gas mixture, and for recovery of a vaporous stream comprising unreacted material of the synthesis gas mixture; PA1 d) a separation zone for separation of material of the synthesis gas mixture into a first hydrogen-rich stream and a second hydrogen-depleted stream comprising carbon oxides and methane; PA1 e) means for supplying at least part of the first hydrogen-rich stream to the steam reforming zone as fuel; and PA1 f) means for recycling at least part of the second hydrogen-depleted stream to the reforming zone for admixture with the vaporous mixture of hydrocarbon feedstock and steam. PA1 a) contacting a vaporous mixture comprising the feedstock and steam in a steam reforming zone with a catalyst effective for catalysis of at least one reforming reaction; PA1 b) recovering from the reforming zone a synthesis gas mixture comprising carbon oxides, hydrogen and methane; PA1 c) supplying material of the synthesis gas mixture to a methanol synthesis zone charged with a methanol synthesis catalyst and maintained under methanol synthesis conditions; PA1 d) recovering from the methanol synthesis zone a product gas mixture comprising methanol and unreacted material of the synthesis gas mixture; PA1 e) supplying material of the product gas mixture to a methanol recovery zone maintained under methanol recovery conditions; PA1 f) recovering from the methanol recovery zone a crude methanol product stream and a vaporous stream comprising unreacted material of the synthesis gas mixture; PA1 g) separating material of the synthesis gas mixture into a first hydrogen-rich stream and a second hydrogen-depleted stream comprising carbon oxides and methane; PA1 h) supplying at least part of the first hydrogen-rich stream to the steam reforming zone as fuel; and PA1 i) recycling at least part of the second hydrogen-depleted stream to the steam reforming zone to form part of the mixture of step a)
Such a plant is described, for example, in WO-A-96/21634.
In order to achieve high yields of methanol, prior art processes have commonly included a recycle loop around the methanol synthesis zone so that unreacted materials leaving the methanol synthesis zone are recycled to the methanol synthesis zone. Thus, U.S. Pat. No. 4,968,722 relates to a process for the production of methanol by reacting carbon monoxide and hydrogen in which the reactants are introduced into a methanol synthesis zone comprising one or more fixed catalyst beds. The effluent from the methanol synthesis zone is fed to an absorption zone where methanol is absorbed. Unreacted reactants are fed to a further methanol synthesis and recovery zone.
U.S. Pat. No. 5,472,986 discloses a methanol production process in which hydrogen is recovered by use of a membrane from a purge gas taken from the methanol synthesis zone. The purged and separated hydrogen is recycled to the methanol synthesis zone as a reactant for methanol synthesis.
U.S. Pat. No. 4,181,675 relates to a methanol synthesis process in which synthesis gas is passed over a methanol synthesis catalyst in a methanol synthesis zone and is then cooled to condense methanol. The remaining gas is recycled to the methanol synthesis zone. A purge stream from this recycle stream may be passed through a membrane to control any build up of inert gases in the recycle stream. Inert materials are separated from carbon oxide and hydrogen, the latter being supplied to the methanol synthesis zone as reactants for methanol synthesis.
DE-A-3244302 discloses a process for the production of methanol in which unreacted methanol synthesis gas is supplied to a three-way separation stage. In the separation stage, CO is separated and recycled to the methanol synthesis zone; CO.sub.2 is separated and supplied to the reforming zone in order to replace part of the water vapour required for reforming; and a residual gas comprising hydrogen, nitogen and methane is supplied to the reforming zone as fuel to heat the reformer tubes.
Various other methanol Production processes are known in the art, and reference may be made, for example, to U.S. Pat. No. 5,063,250, U.S. Pat. No. 4,529,738, U.S. Pat. No. 4,595,701, U.S. Pat. No. 5,063,250, U.S. Pat. No. 5,523,326, U.S. Pat. No. 3,186,145, U.S. Pat. No. 344,002, U.S. Pat. No. 3,598,527, U.S. Pat. No. 3,940,428, U.S. Pat. No. 3,950,369 and U.S. Pat. No. 4,051,300.
A number of different types of reformer are known in the art. One such type is known as a "compact reformer" and is described in WO-A-94/29013, which discloses a compact endothermic reaction apparatus in which a plurality of metallic reaction tubes are close-packed inside a reformer vessel. Fuel is burned inside the vessel, which comprises air and fuel distribution means to avoid excessive localised heating of the reaction tubes. In a compact reformer of this type heat is transferred from the flue gas vent and from the reformed gas vent of the reformer to incoming feedstock, fuel and combustion air. Other types of reformer are not as efficient as the compact reformer in transferring heat internally in this way. However, many other reformer designs are known and some are described in EP-A-0033128, U.S. Pat. No. 3,531,263, U.S. Pat. No. 3,215,502, U.S. Pat. No. 3,909,299, U.S. Pat. No. 4,098,588, U.S. Pat. No. 4,692,306, U.S. Pat. No. 4,861,348, U.S. Pat. No. 4,849,187, U.S. Pat. No. 4,909,808, U.S. Pat. No. 4,423,022, U.S. Pat. No. 5,106,590 and U.S. Pat. No. 5,264,008.
In a conventional plant, synthesis gas is compressed in passage from the reforming plant to the methanol synthesis zone. The synthesis gas compression stage is essentially present in order to provide the required pressure of from 50 bar to 100 bar in the methanol synthesis zone. The synthesis gas compressor is an expensive item which has a significant impact on the overall cost of the plant. Furthermore, the presence in the plant of synthesis gas at such high pressures necessitates the use in the plant of thick walled stainless steel or alloyed steel high pressure pipework. This pipework is expensive to buy, to weld and to use as a construction material. It therefore represents a substantial financial cost in the building of the plant.